Field of the Invention
The present invention relates to a process for spraying one component polyurethane onto a substrate in a manner that is particularly useful for applying adhesives for flexible roof membranes.
Introduction
Spray polyurethane (SPU) foam is available from either one-component (1C) or two-component (2C) SPU foam formulations. Both 1C and 2C SPU formulations are typically available in pressurized vessels and are self-propelled through an applicator when dispensed onto a substrate.
A 1C formulation consists as a single mixture that includes a blowing agent and all reactants except moisture. Prior to use, 1C formulations are maintained under pressure to prevent escape of the blowing agent. 1C SPU formulations typically come in pressurized containers (for example, aerosol-type cans or cylinders) that are pressurized with a blowing agent, which also serves as a propellant that expels the 1C formulation from the pressurized container. To apply a 1C SPU formulation the pressurized container is attached to a nozzle, applicator gun or other dispensing accessory that allows triggered opening of the pressurized container. Upon opening of the pressurized container the 1C SPU foam formulation is self-propelled from the can by the blowing agent through the dispensing accessory. Upon release from the can, the blowing agent also froths the formulation into foam while reactants in the formulation react with atmospheric moisture to cure into a polymeric material. 1C SPU foam formulations provide a particular challenge due to their reactivity with atmospheric moisture. For instance, applicators used to dispense 1C SPU foam formulations tend to plug after use when moisture in the air cures residual 1C SPU foam formulation in the applicator accessory. Therefore, applicators (that is, dispensing accessories) must either be disposed of after use or promptly and extensively cleaned to remove 1C SPU foam formulation so that it does not cure in the applicator.
2C SPU foam formulations consist of two mixtures that are maintained apart until application of the SPU. The two mixtures are typically maintained under pressure with at least one of them containing a blowing agent and both containing a component that acts as a propellant. Upon application of a 2C SPU foam formulation, the pressurized mixtures are self-propelled through an applicator accessory (typically a gun-type applicator) where they are blended together just prior to exiting the applicator. Upon blending together, the two mixtures begin to react to form polymer as the blowing agent froths the reactants. As with the 1C SPU formulations, the applicator used to spray a 2C SPU foam formulation is typically disposed of after use or it must be promptly and thoroughly cleaned to prevent curing of the SPU foam formulation within the applicator. If not promptly and thoroughly cleaned the applicator becomes plugged and non-functional.
One application for SPU foam formulations is as an adhesive for flexible roofing membranes. Flexible roofing membranes are commonly applied over a substrate such as foam insulation board as a barrier material on flat or low-slope roofs. SPU foam formulations are a common adhesive used to adhere a flexible roofing membrane to a substrate in such an application. For example, 1C SPU foam formulations are commonly applied as beads in a pattern on a substrate and then a flexible roofing membrane is applied over the substrates and SPU foam formulation (see, for example, U.S. Pat. No. 5,494,228 and the INSTA STIK™ Roofing STD Technical Data Sheet number 291-92025-0512; INSTA STIK is a trademark of The Dow Chemical Company). The SPU foam formulation beads adhere the roofing membrane to the substrate. However the pattern of SPU foam formulation beads tends to be apparent as ridges in the roofing membrane and some find that undesirable. It is also possible for flexible roofing membranes to bubble or blister between beads of SPU foam adhesive, particularly as temperatures change. The resulting expansion and contraction of the roofing membrane between adhesive beads can serve to undesirably work the membrane free from adhesive beads over time. Therefore, it is desirable to find a way to economically dispense SPU foam formulation over an entire substrate so that there is no longer a bead pattern to impart into the roofing membrane and the entire roofing membrane would be adhered to a substrate.
U.S. Pat. No. 4,996,812 ('812) discloses a method of membrane application in roof construction that includes spraying a layer of adhesive onto a roof substrate and setting a roofing membrane over the adhesive layer. The adhesive layer in '812 can be a 2C SPU foam formulation or a 1C SPU foam formulation. '812 teaching requires an adhesive layer thickness that corresponds to one pound of adhesive per 8 to 12 square feet of substrate surface (408-613 grams per square meter of substrate surface). This is a relatively thick coating of adhesive. Thinner adhesive coatings are more desirable for a number of reasons including less cost and more efficient use of adhesive. Thinner coatings are particularly desirable for 1C SPU foam formulations, which require atmospheric moisture to cure. Moisture takes more time to penetrate into a thick coating so thick 1C SPU foam formulation layers take more time to fully cure than thin 1C SPU foam formulation layers. Longer cure times can result in less homogeneity in the adhesive layer, which can correspond to weaker adhesion. Thinner SPU foam formulation layers can also cure more quickly in low humidity environments because less moisture is needed to penetrate into the coating.
It is desirable to discover a way to apply a thin (less than 400 grams per square meter) layer of a 1C SPU foam formulation on a substrate for use as an adhesive layer for a flexible roofing membrane. It is further desirable to discover a way to apply such a layer of 1C SPU foam formulation in a manner that does not require either prompt and thorough cleaning of the applicator or disposal of the applicator.